Chapter 9 lesson 1 Rocks and weathering
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II.
III.
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Natual processes break down rocks and carry the pieces away.
A. The principle of uniformitaranasm says that the geologic processes that operate today also operated in the
past. So we can infer that ancient landforms formed through the same processes as today.
a. Erosion – the process of wearing down and carrying away rocks
1. Includes the breaking of rocks into smaller pieces and the removal of rock particles by wind, water,
ice or gravity.
b. Weathering – process that breaks down rock and other substances
1. Heat, cold, water, ice, and gases all contribute to weathering.
2. Erosion works all the time to weather and carry away rocks on the Earth.
Forces of weathering can break rocks into pieces. This type of weathering, where a rock is physically
broken, is mechanical weathering. Another type, Chemical weathering breaks down rock through chemical
changes.
A. Mechanical weathering works slowly. But over long periods of time wears away whole mountains.
B. The natural agents of mechanical weathering include freezing and thawing, release of pressure, plant
growth, actions of animals, and abrasion.
a. Abrasion-refers to the wearing away of rock by rock particles carried by water, ice, wind, or gravity.
b. Human activities like mining and farming can also cause weathering.
c. Frost wedging – water seeps into cracks in rocks and freezes (expanding), the ice forces the rock apart.
Wedges of ice in rocks widen and deepen cracks.
C. Chemical weathering can produce new minerals as it breaks down rock.
D. Chemical and mechanical weathering often work together. Chemical weathering creates holes in rock, so
the rock breaks apart easily. As it breaks more surface area is exposed to Chemical weathering.
E. The agents of chemical weathering include water, oxygen, Carbon dioxide, living organisms, and acid rain.
Explain what part each of the following plays in the chemical weathering process.
a. Water – dissolves rock, carries substances that dissolve rock
b. Oxygen – iron and oxygen combine with water (oxidation) this gives us rust and makes rocks crumbly
c. Carbon Dioxide –cause chemical weathering when dissolved in water (carbonic acid)
d. Living organisms – plants roots produce weak acids that dissolve rock
e. Acid Rain – when the air gets polluted they mix with water vapor in clouds, these acids mix with
raindrops and fall as acid rain
The most important factors that determine the rate at which weathering occurs are the type of rock and the
climate.
The minerals that make up the rock determine how fast it weathers. Some rocks weather more easily because
they are permeable.
a. Permeable –means that a material is full of tiny connected air spaces that allow water to seep through
it.
Both chemical and mechanical weathering happen faster in wet climates. Rainfall provides the water needed for
chemical changes and for freezing and thawing. Chemical weathering happens quickly in a hot and wet climate.
NOW ANSWER THIS using the notes you just took:
What breaks down rocks? Be specific. Natural processes of weathering and erosion.
What causes weathering? Erosion works continuously to weather and carry away rock at Earth’s surface. The natural
agents of mechanical weathering include freezing and thawing, release of pressure, plant growth, actions of animals,
and abrasion The agents of chemical weathering include water, oxygen, carbon dioxide, living organisms, and acid rain.
How fast does weathering occur? The most important factors that determine the rate at which weathering occurs are
the type of rock and the climate.
Chapter 9 lesson 2 How soil forms
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B.
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B.
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III.
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Define soil – the loose, weathered material on Earth’s surface in which plants can grow
Soil is a mixture of rock particles, minerals, decayed organic material, water and air.
Bedrock – solid layer of rock beneath the soil.
a. Once bedrock is exposed to air, water, and living things, It breaks down into smaller pieces that are the
most common components of soil.
Gravel, sand, slit and clay (in order from biggest to largest) are the particles in soil that are classified by size
and make up the portions of soil that comes from weathered rock.
The decayed organic material in soil is called humus, it contains nutrients that plants need.
a. The fertility of soil is a measure of how well the soil supports plant growth.
b. Soil texture depends on the size of the soil particles. Loam is soil that is made up of equal parts clay,
sand, and silt. Loam is the best soil for growing plants.
c. Soil can be acidic or basic.
1. pH scale measures acidity.
2. 4 is strongly acidic.
3. 7 is neither acidic nor basic
4. pH greater than 10 is strongly basic.
5. Plants grow best in soil with a pH between 6 and 7.5.
Soil forms as rock is broken down by weathering and mixes with other materials on the surface.
Soil develops layers called horizons.
Soil horizon- is a layer of soil that differs in color, texture, and composition form the layers above or below it
Soil is classified into 3 horizons.
a. C horizon – contains weathered bedrock
b. A horizon – is made up of topsoil (crumbly, dark brown soil that is a mixture of humus, clay, and other
minerals
c. B horizon – subsoil consists of clay and other particles of rock but little humus
Different types of soil support the growth of different types of plants.
Many organisms live in soil. Some soil organisms make humus and other soil organisms mix the soil and
make spaces in it for air and water.
Humus forms in a process called decomposition.
a. Decomposers – are the organisms that break the remains of dead organisms into smaller pieces and
digest them with chemicals
b. This material then mixes the soil as humus. Soil decomposers include fungi, bacteria, worms, and other
organisms.
Earthworms and burrowing mammals, mix humus with air and other materials in soil.
Mammals like mice, moles, and prairie dogs break up hard, compacted soil and mix humus with it.
NOW ANSWER THIS using the notes you just took:
What is soil? Soil is a mixture of rock particles, minerals, decayed organic material, water, and air
How are soil type and plants related? Different types of soil support the growth of different types of plants
How do living things affect soil? Some soil organisms make humus, the material that makes soil fertile. Other soil
organisms mix the soil and make spaces in it for air and water